Improve GetOutboundIP to work without internet access

pkg/common/outbound_ip.go

@@ -2,20 +2,74 @@ package common
 
 import (
 	"net"
-
-	log "github.com/sirupsen/logrus"
+	"sort"
+	"strings"
 )
 
-// https://stackoverflow.com/a/37382208
-// Get preferred outbound ip of this machine
+// GetOutboundIP returns an outbound IP address of this machine.
+// It tries to access the internet and returns the local IP address of the connection.
+// If the machine cannot access the internet, it returns a preferred IP address from network interfaces.
+// It returns nil if no IP address is found.
 func GetOutboundIP() net.IP {
+	// See https://stackoverflow.com/a/37382208
 	conn, err := net.Dial("udp", "8.8.8.8:80")
-	if err != nil {
-		log.Fatal(err)
+	if err == nil {
+		defer conn.Close()
+		return conn.LocalAddr().(*net.UDPAddr).IP
 	}
-	defer conn.Close()
 
-	localAddr := conn.LocalAddr().(*net.UDPAddr)
+	// So the machine cannot access the internet. Pick an IP address from network interfaces.
+	if ifs, err := net.Interfaces(); err == nil {
+		type IP struct {
+			net.IP
+			net.Interface
+		}
+		var ips []IP
+		for _, i := range ifs {
+			if addrs, err := i.Addrs(); err == nil {
+				for _, addr := range addrs {
+					var ip net.IP
+					switch v := addr.(type) {
+					case *net.IPNet:
+						ip = v.IP
+					case *net.IPAddr:
+						ip = v.IP
+					}
+					if ip.IsGlobalUnicast() {
+						ips = append(ips, IP{ip, i})
+					}
+				}
+			}
+		}
+		if len(ips) > 1 {
+			sort.Slice(ips, func(i, j int) bool {
+				ifi := ips[i].Interface
+				ifj := ips[j].Interface
+
+				// ethernet is preferred
+				if vi, vj := strings.HasPrefix(ifi.Name, "e"), strings.HasPrefix(ifj.Name, "e"); vi != vj {
+					return vi
+				}
+
+				ipi := ips[i].IP
+				ipj := ips[j].IP
+
+				// IPv4 is preferred
+				if vi, vj := ipi.To4() != nil, ipj.To4() != nil; vi != vj {
+					return vi
+				}
+
+				// en0 is preferred to en1
+				if ifi.Name != ifj.Name {
+					return ifi.Name < ifj.Name
+				}
+
+				// fallback
+				return ipi.String() < ipj.String()
+			})
+			return ips[0].IP
+		}
+	}
 
-	return localAddr.IP
+	return nil
 }